A variety of opto-electronic devices and systems can be made more efficient by causing light to propagate horizontally inside a substrate containing light-sensitive layers ("horizontally" being relative and meaning that the light has a lengthwise propagation component with respect to the substrate). In many applications, incident light is available from a source normal to the substrate. Light-coupling mechanisms therefore are required substantially to change a propagation direction of the incident light as it enters the substrate. In general, coupling efficiency of the light-coupling mechanism decreases as a change in the propagation direction increases. In addition, the use of substrates with high refractive indices also lowers coupling efficiency.
Many devices, such as photodetectors and other sensors, would benefit from an efficient conversion of normally incident light into horizontally propagating light within the substrate. Normally, as used herein, is relative and means that the light has a propagation component normal to a surface of the substrate. In photodetectors, for example, an interaction length of normally incident photons within an absorbing or sensing layer of the substrate can be drastically increased when the photons propagate obliquely inside the substrate. The resulting increase in an absorption coefficient allows the absorbing layer to be made thinner. Miniaturization of the detector area, coupled with thinner absorbing layers, thus allows for the fabrication of high speed photodetectors.
Some types of photodetectors, such as infrared photodetectors, rely on intersubband transitions in quantum wells. The intersubband transitions, however, can only be induced by a photon electric field directed along a growth direction of the substrate. Since the growth direction is vertical, normally incident light (having an electric field perpendicular to its direction of propagation) produces no interaction between the photon electric field and the quantum wells. The normally incident light therefore must be converted into horizontally propagating light in the substrate to induce the intersubband transitions.
Modulators that exploit the characteristics of bulk polariton effects are another class of devices that would benefit from the horizontal propagation of light inside the substrate. A modulation depth of the modulators increases as the angle of propagation in the substrate approaches horizontal. Additionally, modulation applications require that a beam of light entering a modulation area leaves the modulation area as a beam. Beam characteristics, such as propagation direction therefore must be conserved. Conservation of beam characteristics, however, limits the type of light-coupling mechanisms that may be used with the modulators.
Other devices, such as very sensitive sensors, exploit surface wave characteristics of light. These devices require not only that the light propagate almost horizontally inside the substrate, but also that the propagation direction of the light be conserved.
A number of different light-coupling techniques have been proposed. One technique attempts to couple as much light as possible into the substrate by positioning the substrate under a Brewster angle. While transverse magnetic (TM) polarized light enters the substrate with an efficiency of almost 100%, refraction of the light towards the normal reduces the efficiency of this technique, making it undesirable for detection and sensing applications. Another technique involves polishing the substrate to an angle of less than 45.degree. to convert incident light to an almost horizontally propagating beam. The incident light may thus penetrate the substrate at the proper angle. An efficiency of the polishing technique, however, is low. While the efficiency may be increased by the use of anti-reflective coatings, the polishing technique is difficult to integrate into large arrays.
Diffraction gratings have also been proposed. Diffraction gratings operating in transmission mode are used to diffract light into many orders. While diffraction gratings exhibit a high total efficiency, diffraction efficiencies related to horizontal propagation are low. Single order diffraction gratings, produced from the same material as the substrate and operating in transmission mode, have also been proposed. The gratings, however, are inherently limited in efficiency due to a mismatch between grating modes and substrate modes. The efficiency of the gratings may be increased by covering the gratings with a layer of reflective metal. The reflective layer, however, restrict the grating, allowing it to operate only in reflection mode.
Accordingly, what is needed in the art is a coupling mechanism that operates in transmission mode to more efficiently convert normally incident light into horizontally propagating light while conserving the beam characteristics of the light.